Lamar Odom is fighting for his life after falling into unconsciousness at Dennis Hof‘s Love Ranch South in Pahrump, Nevada … TMZ Sports has learned.

Sources at the Love Ranch tell us … 35-year-old Odom arrived at the Ranch Saturday and was partying with the girls for days. A source at the Ranch said Lamar was taking an herbal substitute for Viagra.

We spoke with Hof … who tells us Tuesday afternoon, a woman went into Odom’s room in the VIP suites and found him unconscious.

We’re told the woman began screaming for the manager — who called 911.

The manager rolled Lamar over on his side and saw “mucus-type liquid coming out of his nose and mouth.”

An ambulance took Lamar to Pahrump hospital, where doctors intubated him … an indication he could not breathe on his own.

We’re told Lamar is being airlifted from the hospital to Las Vegas for more treatment.

Our sources tell us, “It’s not good.”

We’re told Lamar had been partying at the ranch by himself — with no friends or entourage — and is now alone at the hospital.

The ranch staff says they want to be in the room with Lamar but since they’re not family they’re not allowed.

Back in August … Lamar told us how his life had gone downhill since the divorce from Khloe.

UPDATE

11:00 PM PT — Lamar is still in a coma and his heart is failing … our source says he is having serious problems with his kidneys and lungs as well. Khloe Kardashian is currently by his side.

9:30 PM PT — Lamar is still alive, but he is breathing with the assistance of machines and remains in critical condition … according to sources close to the situation.

7:15 PM PT — The Nye County Sheriff says the airlift of Lamar had to be scratched because his 6’10” frame was too big for the chopper. Instead, he was rushed by ambulance to a Las Vegas hospital.

The Sheriff also says the call came in at 3:15 PM for an unresponsive male at the Love Ranch. Paramedics treated Odom and he was taken to a nearby hospital. Sheriff’s detectives conducted an investigation at the brothel. It’s unclear what, if anything, they found on scene.

7:05 PM PT — Sources tell us Khloe and Kim Kardashian … as well as Kris Jenner are heading to Las Vegas shortly to see Lamar at the hospital